The first missile thrown is obviously aimed directly at my head. I raise my arm to deflect it and it clatters hard against the shield I’m carrying, sending a jolt through my shoulder.

Looking up, I can see there are plenty more where that came from. The air is filled with flying wooden bricks – two more smash into me as I try to take in the scene – and a hundred yards ahead, I notice that a bunch of scruffy-looking blokes have supermarket trolleys filled with what look like petrol bombs.

“Dress right!” shouts the police officer standing beside me, waving to indicate to his colleagues and me that under the onslaught of bricks and jeers we’ve let our line of riot shields falter and need to get back into position.

I’m surprised by the violence of our aggressors – all the more so because this is an exercise at the Metropolitan Police’s Public Order Training Centre at Gravesend in Kent. A few weeks ago, I spent a day there to experience the way officers are trained to respond to situations such as Saturday’s protests in central London. While praised for its broadly restrained approach, in the face of intense provocation, the Met has been criticised for failing to do more to prevent the trouble starting. But the line between controlled, effective policing and actions that can lead to accusations of heavy-handedness – or negligence – is difficult to tread.

The Met has come in for a lot of flak over its handling of protesters, and those caught up in protests, in the past few years. The inquest into the death of newspaper vendor Ian Tomlinson, who collapsed at the G20 riots in 2009 after being pushed to the ground by a policeman, began this week. And the Joint Committee on Human Rights has expressed concern over the containment – or “kettling” – of peaceful demonstrators that became a controversial feature of the student riots before Christmas.

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Strategies have adapted accordingly; anyone caught in a kettle will now have a “containment officer”, whose responsibility is to check that the right people are contained. Much of the new thinking is about crowd psychology: using Twitter to dispel false rumours that horses are being brought in to make a charge; and taking care not to run unless necessary, because the sight of a police officer legging it can incite enough panic to turn the mood of a crowd.

The focus is on smoothing edges and trying to introduce a non-confrontational approach; not easy when people tend to respond to a police officer wearing a helmet and carrying a shield as if he is in full battle-dress.

The reality is that all that clobber is disabling, uncomfortable and very hot. It takes three policemen 15 minutes to strap me into the leg pads, arm pads, groin guard, Kevlar-plated stab vest, heavy boots (with steel toe cap and sole, so they can’t be penetrated by broken glass), thick leather gloves, flame retardant trousers and jacket that zip together at the waist, flame retardant balaclava (added after someone ended up with what is wryly described to me as “a very cheap face peel”) and helmet.

I consider myself to be reasonably fit – I ran a marathon last week – but I flunk the “shield run” fitness test that requires officers to jog 500 metres in 2m 45s while wearing this gear and carrying an unwieldy 17lb shield. I feel more clanking medieval knight than crack fighter.

Do people forget it’s actually protective clothing, I ask Commander Bob Broadhurst, head of Scotland Yard’s public order branch when I catch up with him in the Met’s Special Operations Room in Lambeth on the morning of the TUC march. “They do. The dilemma for the commander is if they put the kit on too early it can exacerbate the situation. Too late and officers get hurt.” A recent innovation is to supply officers with a baseball cap to put on as things calm down. “It’s more relaxed, you can hear better, you can see better – it does seem to work,” Commander Broadhurst says.

There is no dedicated force of “riot police”; just officers who all have another job but are trained to level 3 (the most basic, so they know how to set up a cordon or block off a road), 2 (they must pass the shield run test and renew their training with two days every 12 months) or 1 (these will be fitter, have more expertise and are deployed to more contentious areas).

In a fake town at Gravesend they’re taught how to co-ordinate a safe advance through streets in which there’s plenty of trouble, including petrol bombs exploding at our feet. “Now,” shouts an instructor as we kettle some of the brick throwers towards the end of my training day, “lift up your visors and pull down your balaclavas so they can see your faces and try to talk to them calmly and clearly, to explain what’s going on.”

More than the drills it’s the temperamental grounding that is so important. “How to carry the shield is the smallest part of the job. We try to expose people to hostile situations so they can remain calm and controlled and rely on their knowledge and experience when they’re out at work,” says Sergeant Adam Nash from CO11 public order branch. “It’s all about being part of the unit and taking pride in your job.”

In Trafalgar Square on Saturday there was a chance to see how well these exercises would help officers to maintain composure. While paint was thrown and windows smashed on Oxford Street, the day passed fairly quietly here. But at 9.45pm, with only around 300 people left in the square, trouble suddenly flared up around the Olympic clock. “There was a pre-emptive strike from a large section of the crowd who had physically grabbed and tried to drag an officer away from his colleagues,” said Chief Inspector Ian Hackett, who was in charge of the area. “There was a lot of glass thrown in Duncannon Street where they’d gone round the back of one of the restaurants and got bag-loads of recycling to use as ammunition. We had to use batons to drive the crowds back.”

One activist blamed the police for starting the trouble, saying that it was caused by the attempted snatch arrest of a man suspected of damaging a shop front earlier in the day. Either way, scores of reinforcements in protective clothing were brought in, and ugly scenes followed, with bonfires lit and missiles thrown, before the melee eventually subsided with the activists kettled at the foot of Nelson’s Column.

By this time, four or five police officers (who hadn’t been wearing helmets) were in hospital with head injuries and another had a broken collar-bone. Activists in the kettle were protesting at being held and resisting arrest – which created more physical scuffles.

It was after 1am when I talked to Ciara Squires, 18, a student at Queen Mary, University of London, whose friend had just been arrested. “Look at this,” she said, indicating police forming a barrier with their shields, “It’s Orwellian. It’s unacceptable.” Others approached stoic, silent officers, thrusting their faces forward to scream, “We’re doing this to protect YOUR pensions, to protect YOUR children’s education.”

It’s true that the officers peeling protesters off Nelson’s Column, either to arrest them or send them home, were having to be physical, but neither I nor the photographer saw anyone using unnecessary force, despite the frequent screeches of protest from a raggle-taggle bunch of observers. “Most people would be on their chin straps if they had to handle what the officers in Trafalgar Square handled at the end of an 18-hour shift,” says Adam Nash when I discuss it with him later.

“The challenge for us is about finding the balance,” Commander Bob Broadhurst had told me that morning. Back then he had been defending charges of the heavy-handed use of containment to maintain order. But at the end of a long day, as criticism began that the police had not been proactive enough, he must have been reflecting that you’re damned if you do and damned if you don’t.